On 04.12.2010 09:14, jes...@krogh.cc wrote:
There has been a lot discussion about index-only scans and how to make the
visibillity map crash safe. Then followed by a good discussion about hint bits.
What seems to be the main concern is the added wal volume and it makes me
wonder if there is a way in-between that looks more like hint bits.
How about lazily wal-log the complete visibility map say every X minutes or N
amount of tuple updates and make the wal recovery jobs of rechecking visibility
of pages touched by the wal stream on recovery.
If you WAL-log the visibility map changes after-the-fact, it doesn't
solve the race condition we're struggling with: the visibility map
change might hit the disk before the PD_ALL_VISIBLE to the heap page. If
you crash, you can end up with a situation where the PD_ALL_VISIBLE flag
on the heap page is not set, but the bit in the visibility map is. Which
causes serious issues later on.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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